Our long-term goal is to elucidate the mechanism and roles of insulin-degrading processes in health and in the etiology of diabetes mellitus. Insulin in vitro is degraded in a two-step sequential process; first, insulin is split by glutathione-insulin transhydrogenase (GIT) into A and B chains, which are further degraded by a second enzyme system, A/B chains protease(s). Six GITs from four different organs and three different species have been purified. GIT is ubiquitous, its action is probably the first and rate-controlling step in the disposition of insulin, and its concentration (as protein) in liver is under feedback control by the blood insulin level. Recently, a neutral peptidase capable of degrading insulin B chain has been purified to apparent homogeneity. Almost nothing is known about insulin A chain degrading peptidase. Specifically, our objectives are to continue investigations on the structure of GIT, to identify the regulatory mechanisms over the activity of GIT, and to examine the possible physiologic function(s) of GIT and A/B chains protease(s). Work will be carried out to define the biochemical mechanism for the induction of GIT by insulin, to determine the relationship between the insulin metabolic process and the cellular response to insulin, to characterize the B chain degrading neutral peptidase and to purify A chain-degrading peptidase. Human circulating leukocytes contain GIT. Studies, using circulating leukocytes, to compare the insulin degradation in the normal, diabetic, prediabetic, obese, acromagalic and hypopituitary subjects are also proposed.